Since Jesus is the propitiation for the sins of the world, why are not all saved?
The added words “and not for ours only, but also for the sins of the whole world” point out the true scope of Christ’s atoning sacrifice. … Christ’s atoning work has made it possible for any one in the world to be saved, but each sinner must personally accept Christ as his Redeemer (cf. 1 Tim. 4:10) “Men may—yea, and do—reject the propitiation when they reject the Propitiator—the Lord Jesus Christ.”
D. Edmond Hiebert, The Epistles of John, (Greenville, SC: Bob Jones University Press, 1991), 75-76. He quotes W. E. Vine’s Expository Dictionary in the last sentence.